Located only 1,500 light-years away, the Orion Nebula is the brightest diffuse nebula is the sky. This image shows clearly the 3-D structure of this star formation region: a large cavity, created by the radiation pressure from new-born stars located in the brightest area of the image, lies within a huge cloud of dust and gas. Identified as a truly indepedent star cluster, NGC 1980 is associated with this well-studied star formation region, around the brightest star seen at the bottom of this image, Iota Ori. The disks around the star are the result of internal light reflection in the camera optics. // Credit: CFHT/Coelum (J. C. Cuillandre & G. Anselmi)

Using images from the 340-megapixel MegaCam camera on the Canada-France-Hawaii Telescope (CFHT) from the summit of Mauna Kea, astronomers identified the massive cluster of young stars NGC 1980 to be a clearly separate entity from the main cluster of the most-studied star formation region in the galaxy. A technique relying on the combination of optical, infrared, and mid-infrared data ensures astronomers are sampling only stars located in the foreground of the Orion Nebula. This technique also led them to the discovery of a nearby small star cluster, called L1641W.

Astronomers see the Orion Nebula as the benchmark for star-formation studies, a true golden standard, and most of the established measurements of how stars form have been derived from this important region — for example, the distribution of stellar and brown dwarf masses at birth, their relative age, their spatial distributions, and the properties of the planet-forming circumstellar disks surrounding the young stars.

But as it turns out, reality is more complicated. Recent observations with CFHT's MegaCam coupled with previous observations with the European Space Agency’s Herschel and XMM-Newton, NASA's Spitzer and WISE, as well as 2MASS and Calar Alto, revealed the cluster known as NGC 1980 as being a clearly distinct massive cluster of slightly older stars in front of the nebula. Although astronomers knew of the presence of a foreground stellar population since the 1960s, the new CFHT observations revealed that this population is more massive than first thought, and it is not uniformly distributed, clustering around the star Iota Ori at the southern tip of Orion's Sword.

The importance of this discovery is twofold: First, the cluster identified as a separate entity is only a slightly older sibling of the Trapezium cluster at the heart of the Orion Nebula; second, what astronomers have been calling the Orion Nebula Cluster (ONC) is actually a complicated mix of these two clusters.

"We need to refine what we thought were the most robust star and cluster formation observables,” said Hervé Bouy from the European Space Astronomy Center in Madridexplains. He points out the need for a long follow-up work on Orion where "we must untangle these two mixed populations, star by star, if we are to understand the region, and star formation in clusters, and even the early stages of planet formation."

"For me, the most intriguing part is that the older sibling, the Iota Ori cluster, is so close to the younger cluster still forming stars inside the Orion Nebula," said João Alves from the University of Vienna. "It is hard to see how these new observations fit into any existing theoretical model of cluster formation, and that is exciting because it suggests we might be missing something fundamental. Clusters are very likely the favorite mode of star formation in the universe, but we are still far from understanding why that is exactly."

Using images from the 340-megapixel MegaCam camera on the Canada-France-Hawaii Telescope (CFHT) from the summit of Mauna Kea, astronomers identified the massive cluster of young stars NGC 1980 to be a clearly separate entity from the main cluster of the most-studied star formation region in the galaxy. A technique relying on the combination of optical, infrared, and mid-infrared data ensures astronomers are sampling only stars located in the foreground of the Orion Nebula. This technique also led them to the discovery of a nearby small star cluster, called L1641W.

Astronomers see the Orion Nebula as the benchmark for star-formation studies, a true golden standard, and most of the established measurements of how stars form have been derived from this important region — for example, the distribution of stellar and brown dwarf masses at birth, their relative age, their spatial distributions, and the properties of the planet-forming circumstellar disks surrounding the young stars.

But as it turns out, reality is more complicated. Recent observations with CFHT's MegaCam coupled with previous observations with the European Space Agency’s Herschel and XMM-Newton, NASA's Spitzer and WISE, as well as 2MASS and Calar Alto, revealed the cluster known as NGC 1980 as being a clearly distinct massive cluster of slightly older stars in front of the nebula. Although astronomers knew of the presence of a foreground stellar population since the 1960s, the new CFHT observations revealed that this population is more massive than first thought, and it is not uniformly distributed, clustering around the star Iota Ori at the southern tip of Orion's Sword.

The importance of this discovery is twofold: First, the cluster identified as a separate entity is only a slightly older sibling of the Trapezium cluster at the heart of the Orion Nebula; second, what astronomers have been calling the Orion Nebula Cluster (ONC) is actually a complicated mix of these two clusters.

"We need to refine what we thought were the most robust star and cluster formation observables,” said Hervé Bouy from the European Space Astronomy Center in Madridexplains. He points out the need for a long follow-up work on Orion where "we must untangle these two mixed populations, star by star, if we are to understand the region, and star formation in clusters, and even the early stages of planet formation."

"For me, the most intriguing part is that the older sibling, the Iota Ori cluster, is so close to the younger cluster still forming stars inside the Orion Nebula," said João Alves from the University of Vienna. "It is hard to see how these new observations fit into any existing theoretical model of cluster formation, and that is exciting because it suggests we might be missing something fundamental. Clusters are very likely the favorite mode of star formation in the universe, but we are still far from understanding why that is exactly."